Dear Daniele, LF Group,
The high impedance input of your portable should be a good match to the
loop. It may be an advantage to have 2 or 3 turns coupling winding instead
of a single turn, but this is best found by experiment.
It is probably the high impedance tuned winding that could benefit most by
being balanced, rather than the receiver input. The best way to do this is
to make a centre tap in the main loop winding, and connect this to ground.
The tuning capacitor must not be grounded, of course. If you prefer, you
could make a balun with a single centre-tapped winding. The centre tap
would
connect to ground, while the two ends of the balun winding connect to the
two ends of the loop winding. The balun should have much higher inductance
than the loop (at least 5 times, say), or it will significantly increase
the
minimum tuning frequency of the loop. The stray capacitance of the balun
will also reduce the maximum frequency of the loop.
Personally , I have found it makes little difference to pick-up of
interference, whether the loop is balanced or not. It is probably just as
effective to leave the tuned winding floating. It is usually more
effective
to move the loop to a different position (often only a few metres will
make
a big difference) if QRM is a problem. Again, experimentation is the best
way to find out.
Good luck with LF,
Cheers, Jim Moritz
73 de M0BMU
